Method and apparatus for trimming current limit and frequency to maintain a constant maximum power

ABSTRACT

Methods and apparatuses to trim current limit (I p ) and frequency (f) to maintain a constant maximum power. In one aspect of the invention, a power supply regulator having a peak current limit detection threshold and an operating frequency is disclosed. The peak current limit threshold or the operating frequency are adjusted specifically to maintain the I p   m ·f n  product substantially constant from one power supply controller to another where m is substantially equal to 2 and n is substantially equal to 1. In another aspect of the invention, the power supply regulator has a control circuit having a control threshold current. The peak current limit threshold or the operating frequency are adjusted specifically to maintain the product of I p   m ·f n  divided by the control threshold current substantially constant from one power supply controller to another where m is substantially equal to 2 and n is substantially equal to 1.

RELATED APPLICATION

This application claims priority to U.S. provisional application Ser.No. 60/316,583, filed Aug. 31, 2001, entitled “Method And Apparatus ForTrimming Current Limit And Frequency To Maintain A Constant MaximumPower.”

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to power supplies and, morespecifically, the present invention relates to trimming a power supplyfor increased accuracy in the maximum deliverable power of the powersupply.

2. Background Information

Most battery operated portable electronic products such as cell phones,personal digital assistants (PDAs), etc., require a low poweralternating current (AC) to direct current (DC) charger power supplywith a constant voltage and constant current (CC/CV) characteristics forcharging batteries. In known power supplies, the peak current limitthreshold and operating frequency are trimmed independently duringmanufacturing. The two parameters have their own specifications withoutany relationship to each other. Other known power supplies employdiscrete solutions where the tolerances of components like resistors,and capacitors will affect the peak current limit threshold, and theoperating frequency, and by doing so also affecting the maximumdelivered power.

SUMMARY OF THE INVENTION

Methods and apparatuses for trimming a power supply are disclosed. Inone aspect of the invention, a power supply regulator has a peak currentlimit detection threshold and an operating frequency. In the oneembodiment, either the peak current limit threshold or the operatingfrequency are adjusted specifically to maintain the product of thesquare of the peak current limit and the operating frequencysubstantially constant from one power supply controller to another. Inone embodiment, the power supply regulator is used to implement aflyback converter. In another embodiment, the power supply regulator isused to implement a buck converter. In one embodiment, the power supplyregulator operates with a substantially constant frequency under alloperating conditions. In another embodiment, the power supply regulatoroperates with a substantially constant frequency under a fixed range ofoperating conditions.

In another aspect of the present invention, a power supply regulator hasa peak current limit detection threshold and an operating frequency. Thepeak current limit threshold and/or the operating frequency are adjustedduring the manufacture of the power supply controller to maintain theproduct of the square of the peak current limit and the operatingfrequency substantially constant from one power supply regulator toanother. In one embodiment, the power supply regulator is used toimplement a flyback converter. In another embodiment, the power supplyregulator is used to implement a buck converter. In one embodiment, thepower supply regulator operates with a substantially constant frequencyunder all operating conditions. In another embodiment, the power supplyregulator operates with a substantially constant frequency under a fixedrange of operating conditions

In yet another aspect of the present invention, a switched mode powersupply includes a power supply input and a power supply output. A powersupply regulator is coupled to the power supply input and an energytransfer element input. The energy transfer element is further coupledto the power supply output. The power supply regulator has an operatingfrequency and a threshold to detect the peak energy transfer elementinput current. In one embodiment, the operating frequency and/or thepeak current detection threshold of the power supply regulator areadjusted to maintain the product of the square of the peak current limitand the operating frequency substantially constant from one power supplyregulator to another. In one embodiment, the switched mode power supplyis a flyback converter. In another embodiment, the switched mode powersupply is a buck converter. In one embodiment, the power supplyregulator operates with a substantially constant frequency under alloperating conditions. In another embodiment, the power supply regulatoroperates with a substantially constant frequency under a fixed range ofoperating conditions.

In still another aspect of the present invention, a switched mode powersupply includes a power supply input and a power supply output. A powersupply regulator is coupled to the power supply input and an energytransfer element input. In one embodiment, the power supply regulatorhas a fixed operating frequency when the switched mode power supply isproviding substantially maximum output power. In one embodiment, thepower supply regulator has a threshold to detect the peak energytransfer element current. In one embodiment, the operating frequencyand/or the peak current detection threshold of the power supplyregulator are adjusted during the manufacture of the power supplyregulator to maintain the product of the square of the peak currentlimit and the operating frequency substantially constant from one powersupply regulator to another. In one embodiment, the switched mode powersupply is a flyback converter. In another embodiment, the switched modepower supply is a buck converter.

Additional features and benefits of the present invention will becomeapparent from the detailed description and figures set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention detailed is illustrated by way of example and notlimitation in the accompanying figures.

FIG. 1 is a block diagram illustrating one embodiment of a power supplyregulator in accordance with the teachings of the present invention.

FIG. 2 is a diagram illustrating the relationship between the duty cycleand the current (I_(C)) into the control terminal of one embodiment of apower supply regulator in accordance with the teachings of the presentinvention.

FIG. 3 is a diagram illustrating the relationship between the peakcurrent limit threshold I_(p) and the current (I_(C)) into the controlterminal of another embodiment of a power supply regulator in accordancewith the teachings of the present invention.

FIG. 4 is a schematic diagram illustrating one embodiment of a powersupply that has a relatively constant voltage and current characteristicin accordance with the teachings of the present invention.

FIG. 5 a diagram illustrating the relationship between the outputcurrent and output voltage of one embodiment of a power supply inaccordance with the teachings of the present invention.

FIG. 6 is a schematic diagram illustrating another embodiment of a powersupply that has a relatively constant voltage and current characteristicin accordance with the teachings of the present invention

FIG. 7 is a schematic of one embodiment of a trim circuit block used forfrequency trim or peak current limit threshold trim in accordance withthe teachings of the present invention.

DETAILED DESCRIPTION

Embodiments of methods and apparatuses for trimming a power supply forincreased accuracy in the maximum deliverable power of the power supplyare disclosed. In the following description, numerous specific detailsare set forth in order to provide a thorough understanding of thepresent invention. It will be apparent, however, to one having ordinaryskill in the art that the specific detail need not be employed topractice the present invention. In other instances, well-known materialsor methods have not been described in detail in order to avoid obscuringthe present invention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, the appearances of thephrases “in one embodiment” or “in an embodiment” in various placesthroughout this specification are not necessarily all referring to thesame embodiment. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner in one or moreembodiments.

In various embodiments, low cost integrated solutions are provided forcontrolling the maximum deliverable power by trimming of the internallyset peak current limit threshold I_(p), and operating frequency f of apower supply regulator in such a way that the product of the oscillatingfrequency f and the square of the peak current limit threshold I_(p) isa constant or substantially uniform from one regulator to the next.Rather than defining an independent operating frequency f, and anindependent peak current limit threshold I_(p), the two are madedependent on each other, and will together determine the maximumdeliverable power in an AC to DC or DC to DC power supply.

In one embodiment, the maximum deliverable power in a switch mode powersupply that is designed to work in discontinuous operation is determinedby the primary inductance of the transformer, operating frequency f, andpeak current limit threshold I_(p). In one embodiment, to keep themaximum deliverable power substantially constant, the operatingfrequency f and the peak current limit threshold I_(p) are trimmed in away such that the product of Equation 1 below:I_(p) ^(m)·f^(n)   (Equation 1)is substantially constant. In one embodiment, m is substantially equalto 2 and n is substantially equal to 1 such that the product of thesquare of the peak current limit threshold I_(p) and the operatingfrequency f is constant. The operating frequency f and the peak currentlimit threshold I_(p) can be trimmed during manufacturing. Variousembodiments of the present invention can be used to provide a relativelyaccurate, constant voltage and current characteristic for batterychargers.

To illustrate, FIG. 1 is a block diagram illustrating one embodiment ofa power supply regulator 321 in accordance with the teachings of thepresent invention. In one embodiment, the power supply regulator 321 isincluded on a single monolithic chip having as few as three electricalterminals. The power supply regulator 321 includes a power switch 365coupled between drain terminal 323 and source terminal 329. In oneembodiment, power switch 365 includes a metal oxide semiconductor fieldeffect transistor (MOSFET).

As shown in the embodiment depicted in FIG. 1, the power supplyregulator 321 also includes a control current sensor 369 coupled tosense the excess control current through the control terminal 325. Theexcess control current is the total control terminal current less thesupply current of the regulator. The supply current of the regulator isknown. Therefore, the total Control terminal current can be deduced fromthe excess control current. The current received through the controlterminal 325 is responsive to a reflected voltage from an energytransfer element of a power supply in FIG. 4 that the power supplyregulator 321 is coupled to regulate.

In one embodiment, the power switch 365 is switched in response to thecurrent received through the control terminal 325. A control circuit 367is coupled to control current sensor 369 and power switch 365. In oneembodiment, the control circuit 367 modulates the duty cycle and thepeak current limit threshold I_(p) of the power switch 365 to regulatethe output of the power supply in FIG. 4. Accordingly, power switch isswitched on and off by control circuitry 367 to enable and disable thedelivery of power to the output of the power supply to regulate thepower supply output. In so doing, the control circuit 367 modulates theduty cycle of the power switch 365 when the Control terminal currentexceeds a control threshold current. In one embodiment, the controlcircuit 367 reduces the duty cycle when the Control terminal currentexeeds the control threshold current. In one embodiment, the controlcircuit 367 reduces the peak current limit threshold I_(p) when theControl terminal current is below the same control threshold current.

In one embodiment, the current limit comparator 368 compares the currentof the switch 365 to a threshold. When the current of the switch 365exceeds the threshold of the comparator, the switch is turned offimmediately for the remainder of the switching cycle. In one embodiment,the peak current limit threshold trim block 370 sets the peak currentlimit threshold I_(p) of the current limit comparator at the controlthreshold current. The peak current limit threshold I_(p) is reducedwhen the Control terminal current goes below the control thresholdcurrent. In one embodiment, the power supply regulator 321 also includesa start-up circuit 371 coupled to control terminal 325, drain terminal323 and control circuit 367. The power supply regulator 321 alsoincludes an oscillator circuit 373, and a frequency trim circuit 372 toprogram the operating frequency in the oscillator circuit 373. In oneembodiment, oscillator 373 is coupled to provide an oscillating signalhaving the operating frequency f, which is received by control circuit367 to switch power switch 365. In one embodiment, power switch 365 isswitched in response to the oscillating signal received from oscillator373.

FIG. 2 shows a diagram illustrating the relationship between the dutycycle and the current (I_(C)) into the control terminal of oneembodiment of power supply regulator 321. The diagram shows twodifferent regions of operation. In the first region, the current (I_(C))into the control terminal is lower than the Control threshold currentand the duty cycle is maximum, and the power supply regulator 321operates in current limit mode. In this mode, the peak current limitthreshold I_(p) is reduced to regulate the current at the output of thepower supply. In the second region with increased current (I_(C)) intothe control terminal output voltage is controlled by regulating the dutycycle.

FIG. 3 shows a diagram illustrating the relationship between the peakcurrent limit threshold I_(p) and the current (I_(C)) into the controlterminal of one embodiment of power supply regulator 321. The peakcurrent limit threshold I_(p) increases with increased current (I_(C))into the control terminal of the power supply regulator 321.

FIG. 4 shows one embodiment of a flyback power supply that has arelatively constant voltage and current characteristic. The feedbackinformation is provided to the power supply regulator 321 at its controlterminal. The current at the control terminal is proportional to thevoltage across resistor 235, which in turn is responsive to the outputvoltage 200. The power supply regulator 321 reduces the duty cycle ofthe integrated power metal oxide semiconductor field effect transistor(MOSFET) included in power supply regulator 321 when the voltage acrossresistor 235 increases above the voltage necessary to establish aControl threshold current. In this section, the output is in constantvoltage mode. The power supply regulator 321 reduces the peak currentlimit threshold I_(p) of the integrated power MOSFET when the voltageacross resistor 235 decreases below the voltage necessary to establishthe Control threshold current. The peak current limit threshold I_(p) isreduced as a function of the voltage across resistor 235 to regulate theoutput load current.

FIG. 5 is a diagram illustrating the relationship between the outputcurrent and output voltage of one embodiment of a power supply inaccordance with the teachings of the present invention. As can be seenin curve 400, the power supply exhibits a substantially constant outputcurrent and voltage characteristic. That is, as output currentincreases, the output voltage remains substantially constant until theoutput current reaches an output current threshold. As the outputcurrent approaches the output current threshold, the output voltagedecreases as the output current remains substantially constant over thedrop in output voltage. It is appreciated that the constant outputvoltage and constant output current characteristics of the presentinvention are suitable for battery charger applications or the like.

FIG. 6 shows one embodiment of a buck converter power supply 601 thathas a relatively constant voltage and current characteristic. Thefeedback information is provided to the power supply regulator 321 atits control terminal. The current at the control terminal isproportional to the voltage across resistor 603, which in turn isresponsive to the DC output 600 voltage. The power supply regulator 321reduces the duty cycle of the integrated power metal oxide semiconductorfield effect transistor (MOSFET) included in power supply regulator 321when the voltage across resistor 603 increases above the voltagenecessary to establish the Control threshold current. In this section,the output is in constant voltage mode. The power supply regulator 321reduces the peak current limit threshold I_(p) of the integrated powerMOSFET when the voltage across resistor 603 decreases below the voltagenecessary to establish the Control threshold current. The peak currentlimit threshold I_(p) is reduced as a function of the voltage acrossresistor 603 to regulate the output load current.

FIG. 7 is a schematic of one embodiment of a trim circuit block used forfrequency trim or peak current limit threshold I_(p) trim in accordancewith the teachings of the present invention. In this circuit, I_(ref) isa reference current source, and I_(trim) is the trimmed current source.I_(trim) is a current mirror of I_(ref). The ratio of the current mirrorchanges as more zeners are trimmed. For example when zener 750 is nottrimmed, transistor 730 will be practically open since the voltage dropof the zener will inhibit any conduction through the transistor. Whenzener 750 is trimmed (shorted), transistor 730 will conduct and be inparallel to transistor 720, thereby changing the ratio of the currentmirror.

For the oscillator frequency block, I_(trim) can be used to charge anddischarge a capacitor. For the current limit comparator, I_(trim) can beused to set the threshold of the comparator. In one embodiment, the peakcurrent limit threshold I_(p) can be trimmed first. Then the targetfrequency or operating frequency f can be adjusted by the measuredcurrent limit so that the product of the peak current limit thresholdI_(p) squared and operating frequency f will remain constant. Thismethod will provide a constant deliverable power to the output load ofthe power supply.

In yet another embodiment, the operating frequency f can be adjustedbased on the peak current limit threshold I_(p) and control thresholdcurrent so that the product of I_(p) ^(m)·f^(n) divided by the controlthreshold current is constant. Since the control threshold currenttracks the output voltage of the power supply, this method essentiallycompensates the deliverable power for the output voltage, resulting inconstant power supply output current at the maximum output power pointon the power supply output characteristic.

In the foregoing detailed description, the method and apparatus of thepresent invention has been described with reference to specificexemplary embodiments thereof. It will, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

1-32. (canceled)
 33. A circuit, comprising: control circuitry coupled toa power transistor; an oscillator coupled to the control circuitry, thecontrol circuitry to switch the power transistor in response to theoscillator; current limit circuitry coupled to the power transistor andthe control circuitry to limit a current through the power transistor;and trim circuitry coupled to the oscillator and the current limitcircuitry, the trim circuitry to be trimmed during manufacture of thecircuit to determine a maximum deliverable power value of a power supplysuch that the maximum deliverable power of the power supply issubstantially constant with a maximum deliverable power of another powersupply.
 34. The circuit of claim 33 wherein the trim circuitrycomprises: frequency trim circuitry coupled to the oscillator; and peakcurrent limit threshold trim circuitry coupled to the current limitcircuitry.
 35. The circuit of claim 34 wherein the an operatingfrequency f of the oscillator is determined in response to the frequencytrim circuitry and a peak current limit threshold (I_(p)) is determinedin response to the peak current limit threshold trim circuitry.
 36. Thecircuit of claim 35 wherein the frequency trim circuitry and the peakcurrent limit threshold trim circuitry are trimmed during manufacture ofthe power supply such that a product of a square of the peak currentlimit threshold I_(p) and the operating frequency f is substantiallyconstant with a product of a square of a peak current limit thresholdI_(p) and an operating frequency f of another power supply.
 37. Thecircuit of claim 33, wherein the circuit is included in a flybackconverter power supply.
 38. The circuit of claim 33 wherein the circuitis included in a buck converter power supply.